The invention relates to scanning probe microscopy. In particular, the invention relates to probes for use in scanning probe microscopy.
Complete bibliographic citations to the references discussed herein are contained in the Bibliography section, directly following the Detailed Description.
A scanning probe microscope (SPM) uses a probe to scan the surface of a sample and provides a three-dimensional image of atoms or molecules on the surface of the object. The probe is an extremely sharp point that can be as narrow as a single atom at the tip. There are several different types of scanning probe microscopy, including scanning tunneling microscopy (STM), scanning force microscopy (SFM), atomic force microscopy (AMF), magnetic force microscopy (MFM), and magnetic resonance force microscopy (MRFM).
Carbon nanotube tips (CNTs) offer many advantages over the standard SFM probes, namely high aspect ratio, high resolution, durability, minimal tip or sample damage and, perhaps most important, tailoring. CNTs for SFM are described, for example, in WO 98/05920, published Feb. 12, 1998. CNTs as probes for surface metrology are very useful. Their high-aspect ratio enables profiling morphologies that are inaccessible to conventional probes.
Since the commercialization of SFM, a variety of probes have been developed to meet specific sample characterization requirements. The work of Dai et al. (1) introduced a novel probe: CNTs. Although CNTs are not widely used, they offer many advantages over the standard SFM probes. The potential of CNTs as probes for metrology rests on their very desirable properties, including the following. First, because they are long tubes (tubes with lengths from nanometers to several micrometers can be fabricated as metrology probes), they have a high aspect ratio. Thus, even if there were no enhancement in resolution (i.e., the nanotube end form relative to that of conventional probes) metrology probes using nanotubes can profile morphologies that are inaccessible to conventional probes.
Second, the end form of nanotubes or nanotube bundles is invariably smaller than 20 nm diameter, and can be made as small as 3 nm, providing resolution of this order. Whereas a previously un-used conventional tip can achieve 20 nm resolution, it rapidly degrades within two or three scans. Resolutions better than 20 nm are not routinely obtainable with conventional tips.
Third, the mechanical properties of the tubes are such that they do not break or wear. Even if the tubes were to provide no improvement in resolution, this feature is essential for several reasons: 1) no change in resolution over time, unlike other probes presently in use; 2) reliability-no debris left in any of the regions that have been probed, unlike other probes; 3) reduced down time for probe changes, a significant advantage for a real-time, on-line tool, such as in process inspection and characterization of advanced microelectronic device structures.
Fourth, the mechanical properties (i.e., compliancy) of the probe are adjustable because tubes can be fabricated in bundles of various sizes. The resolution need not be degraded, as is described below. In many cases a stiffer probe is advantageous.
Additionally, short nanotubes may also have applications in direct-contact nanolithography and microscopy. Furthermore, one third of nanotubes, and of bundles likely more, are conducting. Hence, in addition to topographic and morphological imaging, there are opportunities to use nanotube probes for electrical measurements with nanometer-scale lateral resolution, for example in capacitance/voltage (CV) profiling of surfaces.
The current most common method for CNT fabrication consists of manually attaching the carbon nanotubes onto commercial SFM tips using a thin adhesive, such as scanning electron microscope (SEM) carbon tape. Alternative methods, in which the nanotubes are grown directly onto SFM tips using catalytic methods, have recently been demonstrated by various researchers (2, 3).
The attached nanotubes can be multi-wall nanotubes (MWNT) or single-wall nanotubes (SWNT). Most of the CNTs produced by attachment consist of a bundle of MWNTs or SWNTs that is often too long to function in SFM. The length of the CNT can be tailored to the desired value by applying voltage between the tip and the sample in the SFM. A voltage of 5-10 V is normally adequate to xe2x80x9cburnxe2x80x9d part of the bundle, thereby shortening the length of the bundle. However, when using this process, the geometry at the working end of the shortened nanotube bundle is arbitrary because the xe2x80x9cburningxe2x80x9d procedure is uncontrolled. While a resolution of 20 nm to 50 nm is achieved very easily using the convention route of CNT shortening, further improvements in resolution using CNTs are needed. This requires developing a technique that controllably exposes only one nanotube from the bundle, such as the method described herein.
The invention, which is defined by the claims set out at the end of this disclosure, is intended to solve at least some of the problems noted above. A method is provided that involves fabrication of carbon nanotube tips (CNTs) using a thin adhesive for attaching the carbon nanotubes onto commercial scanning force microscopy (SFM) tips or multi-wall carbon nanotubes (MWNT).
Described herein is a method for controlling the end-form of a nanotube bundle so that a single nanotube protrudes from it. In the preferred embodiment, the nanotube bundle is mounted on a silcon tip, such as a silicon whisker attached to a cantilever. This improvement of resolution with CNTs relies on techniques that controllably expose one nanotube from the bundle. In the case of SWNT bundles, this improves the resolution by an order of magnitude, because the diameter of a SWNT is approximately 1 nm.
An equally important issue is throughput, which is related to the speed of imaging. More effort needs to be directed at improving the speed of operation in the intermittent-contact mode (4). This mode will be necessary in most high-performance profiling and will most likely be an absolute requirement when high-aspect-ratio features (e.g., deep holes in next-generation device fabrication) must be probed, because the probe will have to be long and narrow and changes in morphology will occur suddenly. Increased speed will require fast resonator/sensor modules, which demand mechanical structures with higher resonant frequencies and high values of Q and also better driving and feedback electronics. (Q is a measure of the ability of a system with periodic behavior to store energy as compared to the energy dissipated per cycle.) Higher speed will be essential if critical-dimension tools and profilometers are to become routine in process inspection and monitoring in the semiconductor device industry.
Also described herein is the combination of tuning forks (5) and nanotubes for use as probes for SPM. The nanotubes are mounted on the tuning forks using the same method as for regular Si cantilevers.
Further advantages, features, and objects of the invention will be apparent from the following detailed description of the invention in conjunction with the associated drawings.